1. Field of the Invention
This invention relates to a liquid-crystal display device of twisted nematic (TN) type with improved multiplex driving characteristics.
2. Description of the Related Art
A conventional TN-type liquid-crystal display device is shown e.g., by FIG. 6. In this figure, the nematic liquid-crystals sandwiched between the two electrode substrates (a and b) having the transparent electrodes (c and d) thereon, are oriented into the twisted helical structure by the orientation membranes (e and f) being made of an insulating material and treated by the so-called rubbing method in which the membrane surfaces to be kept in contact with the liquid crystals are each rubbed unidirectionally. If the two electrode substrates, a and b, are faced each other so that the orientation direction of the membranes will be perpendicular to each other as shown in FIG. 7 and nematic liquid-crystals having a positive dielectric anisotropy (N.sub.p) are sandwiched between the two electrodes, then the liquid-crystal molecules will take a helical structure twisted 90.degree.. In FIG. 7, j is the rubbing direction on the upper membrane e, while k is the rubbing direction on the lower membrane f. On the surfaces of the liquid-crystal cell thus constructed, are provided polarizing plates, h and i, so that the polarization axis 1 on plate h and the polarization axis m on plate i are conformable to the orientation directions of membranes e and f, respectively (refer to FIG. 7).
When a matrix-type liquid-crystal display device comprising a number of transparent electrodes is driven by the optimum voltage equalization method, the ratio of the effective voltage at turned-on picture elements V.sub.rms (ON) to that at turned-off picture elements V.sub.rms (OFF) is represented by the following equation: ##EQU1##
If N.fwdarw..infin., then .alpha..fwdarw.1; that is, the difference between V.sub.rms (ON) and V.sub.rms (OFF) decreases with increasing N. This indicates that liquid crystal materials having sharper response at threshold voltage and orientation membranes having improved properties are required for the development of matrix-type, liquid-crystal display devices with a large amount of information to be displayed which show a high contrast ratio even with a small difference between V.sub.rms (ON) and V.sub.rms (OFF). In recent years, there has been an increasing demand for a larger amount of information to be displayed. However, conventional TN-type liquid-crystal display devices as described above can hardly respond to the increase in N.
An attempt has been made to improve the multiplex driving characteristics by setting the twist angle of liquid crystal molecules at a level higher than 90.degree.. This may be effected by adding, to nematic liquid crystals, a larger amount of an optical active substance that induces twisted helical structure. The amount of said optical active substance to be added should preferably be such that the ratio of helical pitch of liquid crystal molecules (p) to the thickness of liquid-crystal layer (d) will satisfy d/p.congruent..phi./360.degree., and is usually set to satisfy (.phi./360.degree.-1/4)&lt;d/p&lt;(.phi./360.degree.+1/4).
However, such liquid-crystal display devices containing a larger amount of optical active substance have the problem that the liquid crystal molecules tend to become disoriented upon voltage application to form domains, resulting in scattering of light and lowering of the contrast ratio. To avoid this disadvantage, it has been attempted to form the orientation membrane of SiO.sub.2 by the oblique deposition method, thereby ensuring a larger inclination of the liquid crystal molecules being in contact with the membranes, that is, a larger pretilt angle. With the oblique deposition method, however, it is not easy to form a uniform orientation membrane over a wide area, and hence manufacture of liquid-crystal display devices of larger areas is very difficult. In addition, it is difficult to simultaneously perform uniform orientation operations on a larger number of electrode substrates, making mass-production impracticable.
In this connection, we formerly found that domain formation can be diminished within the range of .phi..ltoreq.300.degree., if a N.sub.p -type nematic liquid crystal having a large positive dielectric anisotropy is selected from compounds of biphenyl, pyrimidine, dioxane and ester types and if the helical pitch of liquid crystal molecules (p) and the thickness of liquid-crystal layer (d) are set so that the following relationship will be satisfied (Japanese Unexamined Patent Pub. Nos. 50812/1987 and 194224/1987): EQU (.phi./360.degree.-1/4)&lt;d/p&lt;(.phi./360.degree.+1/4)
In this case, another problem arises from the large refractive index anisotropy (.DELTA.n: 0.20 to 0.25) of the above N.sub.p -type nematic liquid crystals. Since the value of .DELTA.n.d should preferably be set in the range of 0.5 .mu.m.ltoreq..DELTA.n.d .ltoreq.1.1 .mu.m in the viewpoint of image quality of display devices, the thickness of liquid-crystal layer (d) should be set at a level of 5.5 .mu.m or less. In such thin cells, however, conductive impurities unavoidably involved therein tend to cause a conductive state across the two electrodes, thus adversely affecting correct display, and increasing the thickness of liquid-crystal layer to avoid this trouble leads to degraded image quality.
Further studies to overcome these difficulties have led us to find that liquid-crystal display devices of high image quality free from domain formation up to an angle (.phi.about 300.degree. can be obtained, if an N.sub.p -type liquid-crystal composition comprising a specific pyrimidine derivative with a low positive dielectric anisotropy is used and if the helical pitch of liquid crystal molecules (p) and the thickness of liquid-crystal layer (d) are set as described above.